In image forming devices such as an electrophotographic device and an electrostatic recording device, an electrostatic latent image, which is formed on a photosensitive member, is firstly developed with a toner. The formed toner image is then transferred to a transfer material such as paper as necessary, and then fixed by various means such as heating, pressurization or solvent vapor.
In such image forming devices, digital full color copying machines and digital full color printers have begun to be put into practical use. In a digital full color copying machine, a colored image manuscript is subjected to color separation by means of blue, green and red filters, an electrostatic latent image that corresponds to the original colored manuscript and is formed of dots each having a diameter of from 20 to 70 μm is developed by using respective yellow, magenta, cyan and black toners, and a full color image is formed by utilizing subtractive color mixing. Toners for full color having respective colors require similar degrees of tinting powers so that reproduction of accurate tincture is enabled, but among full color toners, yellow toners specifically had a problem of low tinting power.
Therefore, in order to improve tinting power, as a method for selecting a yellow pigment, for example, a method for measuring the interfacial tension with respect to water of a liquid in which a pigment is dispersed in styrene, as described in Patent Literatures 1 and 2, has been suggested.
Patent Literature 1 describes that a polymerizable monomer composition containing a polymerizable monomer (for example, styrene), a yellow pigment, a wax, a resin a (for example, a vinyl-based copolymer) and a resin b (for example, a polyester resin) is used for the production of a yellow toner. In Patent Literature 1, upon the synthesis of toner particles by forming oil droplets of the polymerizable monomer composition in an aqueous medium, and polymerizing the polymerizable monomer, the interfacial tensions (mN/m) of the dispersion of the respective materials in styrene or of styrene with respect to water are defined as follows.Interfacial tension (styrene)>interfacial tension (yellow pigment)>interfacial tension (resin b)  Formula IInterfacial tension (styrene)>interfacial tension (resin a)>interfacial tension (resin b)  Formula II0≤|interfacial tension (yellow pigment)−interfacial tension (resin a)|≤10.0  Formula III5.0≤interfacial tension (styrene)−interfacial tension (resin b)≤17.0  Formula IV
In the invention of Patent Literature 1, the purpose of the selection of the materials so as to satisfy the Formulas I and II is to form an outermost shell by the resin b in each of the oil droplets of the polymerizable monomer composition. Furthermore, in the invention, the purpose of the selection of the materials so as to satisfy the Formulas III and IV is to enhance the dispersibility of the yellow pigment by the resin a in the oil droplets of the polymerizable monomer composition.
Patent Literature 2 describes that a polymerizable monomer composition containing a polymerizable monomer (for example, styrene), yellow colorants (a yellow pigment and C. I. Solvent Yellow 98) and a wax is used. In Patent Literature 2, upon the synthesis of toner particles by forming oil droplets of the polymerizable monomer composition in an aqueous medium, and polymerizing the polymerizable monomer, the interfacial tensions (mN/m) of the respective materials in the yellow colorants with respect to water are defined as follows.3.0≤(B−A)≤15.0  Formula V
(In the Formula V, the interfacial tension A represents the interfacial tension with respect to water of a dispersion in which the yellow pigment is dispersed in styrene, and the interfacial tension B represents the interfacial tension with respect to water of a solution in which C. I. Solvent yellow 98 is dissolved in styrene.)
In the invention of Patent Literature 2, the purpose of the selection of the yellow colorants so as to satisfy the Formula V is to maintain the dispersion state of the yellow pigment until the polymerization reaction of the polymerizable monomer in the oil droplets is completed, without causing layer separation between the yellow pigment and the dye (C. I. Solvent Yellow 98) in the oil droplets of the polymerizable monomer composition and aggregation of the yellow pigment in the oil droplets.